Various kinds of spectroscopic analysis using a semiconductor laser as the light source have been used. The characteristics common to analysis in the prior art are that a matrix gas must be transparent at wavelengths of the light source and that the intensity of the light may be attenuated by absorption by impurity molecules to be measured. With the analysis of the prior art, identification of trace amounts of an impurity can be made from the difference between absorption intensities (amounts of absorption) of a matrix gas and a target impurity, at identical wavelengths.
However, most gas molecules have overtone absorption bands and combination tone absorption band in the near infrared wavelength region. When both the matrix gas and the target impurity to be measured absorb light at identical wavelengths and the amount of optical absorption by the matrix gas is greater than that by the target impurity, the resolution and the accuracy of the measurement decrease because of interference absorption.
There was also a problem in that noise from the light source must be decreased in order to improve detection sensitivity. In the prior art, a dual beam measurement method is employed to decrease the noise. However, with this method, only the noise from the light source can be eliminated whereas influences due to the interference absorption arising from the matrix gas and to absorption arising from the target impurity in a purge box (out of a cell) cannot be eliminated.
There are two methods to eliminate noise in the dual beam measurement method of the prior art. In one conventional method, powers of beams reaching two light detectors are adjusted to be balanced, photoelectric conversion is carried out, and noise is thereafter cancelled with a differential amplifier. In another conventional method, by amplified gains from current/voltage conversion preamplifiers, which are provided after light detectors and before lock-in amplifiers, are adjusted so that outputs from the preamplifiers in two channels are balanced, and noise is thereafter cancelled by a differential amplifier. With these methods, noise cannot be completely removed and the adjustment for the apparatus is complicated because these methods make control of analog signals in a time domain delicate.
FIG. 3 shows an example of an analysis apparatus of the prior art, which comprises a semiconductor laser 1 (a light source), a collimating lens 11, a half mirror 12, a mirror 13, a light splitting means 4 for splitting light from the semiconductor laser 1 into two beams, a sample cell 2 through which a split beam passes, a first detector 5 for measuring an intensity of the beam, a second detector 6 through which the other split beam passes, first and second lock-in amplifiers 21 and 22, first and second AD converters 23 and 24 which are connected to the first and second lock-in amplifiers 21 and 22, and a computer 25 for identifying a trace impurity concentration in the matrix gas in the sample cell 2. A laser current driver 1a drives the semiconductor laser 1 and a gas supply means 9 introduces and exhausts the matrix gas into and from the sample cell 2.
FIGS. 4 to 7 show the result of the measurement using the analysis apparatus of the prior art. FIG. 4 shows absorption spectra of NH.sub.3 and H.sub.2 O around a wavelength of 1.37 .mu.m, and FIG. 5 shows absorption spectra of SiH.sub.4 and H.sub.2 O around a wavelength of 1.38 .mu.m. As seen from FIGS. 4 and 5, the measurements for trace moisture concentrations in NH.sub.3 and SiH.sub.4 interfere with the absorption spectra of NH.sub.3 and SiH.sub.4.
FIG. 6 shows spectra obtained by measurement in the case where H.sub.2 O is added to the NH.sub.3 gas, and FIG. 7 shows a calibration curve obtained from the result of the measurement of FIG. 6. It is obvious from FIG. 7 that the absorption spectrum peak corresponding to the concentration of H.sub.2 O is not linearly varied because of the interference absorption of NH.sub.3.
With the analysis method of the prior art, the influences due to the interference absorption arising from the matrix gas and to absorption arising from the target impurity in a purge box (out of a cell), cannot be eliminated. As a result, it is increasingly demanded that the influences from the interference absorption of the matrix gas, from the noise of the light source, and from the absorption of the target impurity in the purge box (out of the cell), be simultaneously removed in identification of impurities in gas having interference absorption, such as NH.sub.3 and SiH.sub.4, with high sensitivity and accuracy.